Just like we draw FBD of a block on table or on land.How should we approach the FBD for earth. enter image description here Since there is mg downward and mg upward in interaction with block and earth.What will be with earth ?can we create normal reaction between the block and earth in FBD of earth.

Is the mg of earth the force of block on earth I think ?

  • 1
    $\begingroup$ You can draw the reaction force at the surface upwards, it applies to the block. At the surface downwards, it applies to the Earth. That would depend on what you study, perhaps it is enough to cause the soil to sink. If you are concerned with the Earth motion and assume it to be rigid, it can be displaced to the Earth center of mass. Latter case is unlikely. $\endgroup$
    – Alchimista
    Dec 11 '20 at 11:05
  • $\begingroup$ In think your question has been answered.Just stopped to say that it is an interesting question. $\endgroup$
    – srijan Sri
    Dec 26 '20 at 13:42

Newton's law of gravity tells us that if the gravitational attraction of body A acting on body B is $\vec F$ then the gravitational attraction of body B acting on body A is $-\vec F$. In other words, it has the same magnitude but the opposite direction.

So if the Earth attracts the block with a force $mg$, the block also attracts the Earth with a force $mg$ in the opposite direction. When you throw a ball in the air, the ball attracts the earth with the same force that the Earth attracts the ball. But the mass of the Earth is so large that its acceleration is negligible, so usually we can safely ignore the motion of the Earth.

If the block is standing on the ground then a free-body diagram of the Earth (showing only the forces due to the block) would show an upwards force $mg$ due to the gravitational attraction of the block, balanced by a downwards force $N=mg$ which is the normal force that the block exerts on the Earth.

  • $\begingroup$ Ok.Would there be any gravitational forces by other planets like moon? $\endgroup$
    – srijan Sri
    Dec 11 '20 at 14:23
  • $\begingroup$ N will not be equal to mg right because earth does have some acceleration but it just that we can’t feel it or not visible $\endgroup$
    – srijan Sri
    Dec 11 '20 at 14:29
  • $\begingroup$ @robertpatrick If the block is on the ground and the Earth exerts a normal force $N=mg$ upwards on the block then Newton's third law tells us that the block exerts an equal and opposite normal force $N=mg$ downwards on the Earth. If the normal forces were not equal and opposite then the block would be in equilibrium but the Earth would not - which is not logically possible if they stay in contact $\endgroup$
    – gandalf61
    Dec 11 '20 at 15:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.